Hydrothermal synthesis of Mn3O4 nanorods modified indium tin oxide electrode as an efficient nanocatalyst towards direct urea electrooxidation

Control fabrication of metal-oxide nanocatalysts for electrochemical reactions has received considerable research attention. Here, manganese oxide (Mn3O4) nanorods modified indium tin oxide (ITO) electrodes were prepared based on the in-situ one-step hydrothermal methods. The nanorods were well characterized using field emission scanning electron microscopy, Fourier transform infrared, and X-ray diffraction spectroscopy. The results showed the formation of pure crystalline Mn3O4 nanorods with a length of approximately 1.4 μm and a thickness of approximately 100 ± 30 nm. The Mn3O4 nanorod-modified ITO electrodes were used for accelerating urea electrochemical oxidation at room temperature using cyclic and square wave voltammetry techniques. The results indicated that the modified electrode demonstrated excellent electrocatalytic performance toward urea electrooxidation in an alkaline medium over concentrations ranging from 0.2 to 4 mol/L. The modified electrode showed high durability, attaining more than 88% of its baseline performance after 150 cycles; furthermore, the chronoamperometry technique demonstrated high stability. Thus, the Mn3O4 nanorod-modified ITO electrode is a promising anode for direct urea fuel cell applications.


Introduction
While fossil fuel as an energy source is an important pillar of global economic development, it does not promote human existence because of its associated high environmental pollution [1]. Consequently, searching for alternative and sustainable energy sources has become a global goal to maintain current energy consumption and prevent environmental catastrophes [1][2][3][4][5][6][7].
Owing to the several advantages of the fuel cells including their environmentally friendly, and high efficiency. Thus, fuel cells are among the uppermost other renewable sources, including solar, wind, and hydropower energies [7][8][9]. Hydrogen is considered the future fuel of our economy and life [7]. However, many challenges have evolved in hydrogen production, storage, and transportation [1]. Accordingly, several liquid fuels, including methanol, ethanol, and formic acid, have been used for electricity production, with significant advantages [10]. However, the use of these fuels has increased carbon emissions. Thus, nitrogen-based fuels such as ammonia and hydrazine have been used to generate hydrogen gas using thermal, catalytic, or electrolytic methods, which are promising carbon-free energy sources [11,12]. Furthermore, N-based fuels are directly used to produce electricity without any preconversion [13,14].
Although, liquid fuels have advantages owing to their high energy density and convenience to stockpile at low costs compared with gaseous fuels. However, liquid fuels suffer many challenges regarding safety, including toxicity [15] and volatilization. Thus, the use of solid fuels with high energy densities could be a potential solution to these challenges [16,17]. Urea is a well-known fertilizer [8, 18, 19], with a considerable energy density of 16.9 MJ/L, which is 10 times greater than that of hydrogen [8,18]. Green hydrogen is produced by water electrolysis, requiring an overall cell voltage of 1.23 V [20]. While green hydrogen production based on electrolyzed urea needs only 0.082 V, saving energy and cost [20], urea produces 10 wt% valid hydrogens.
The uses of urea as a solid fuel have additional advantages, including (1) high energy density; (2) high solubility; (3) the manufacturing of urea, including the use of CO 2 , making it a CO 2 -neutral energy source; (4) a solution for environmental issues resulting from the use of urea as a fertilizer or urine; (5) a potential solution for treated nitrogen-rich wastewater using urine as a direct fuel; (6) high stability; (7) relatively non-toxic; (8) a safe and easily transported carrier; and (9) low cost. These advantages promote urea as the best sustainable hydrogen carrier and provide a sustainable energy supply [21][22][23][24].
Industrial and municipal wastewater, especially those containing urea, must be treated for environmental protection and energy production [8,9]. Thus, urea can be electrooxidized to generate electricity while purifying wastewater simultaneously [7,8,24].
The first direct urea fuel cell (DUFC) was developed in 2010 [25]. Alkaline urea electrooxidation has been reported as the best method for urea-containing wastewater treatment and urea electricity generation [23]. The electrooxidation of urea in an alkaline medium has been shown in Eqs (1-3) [26]. Typically, electrooxidation of the fuel (urea) includes a reaction with the supporting electrolyte (KOH) in certain proportions. Two reaction mechanisms based on the use of urea have been proposed: (i) the electrochemical reaction when urea is applied as a fuel for DUFC, as shown in Eqs (1-3), and (ii) the use of urea for the electrolytic production of hydrogen, as shown in Eqs (4)(5)(6). The anodic and cathodic reactions of urea indicated that the molar ratio of KOH should be approximately 8 or 6 times that of urea [27].
Anodic reaction : Cathodic reaction : Overall reaction : Anodic reaction : Cathodic reaction : Overall reaction : Recently, several metal oxide nanomaterials were reported as electrocatalytic materials for enhancing energy storage and production [29][30][31][32]. To reduce the cost of fuel cells, research has focused on developing transition metal-modified electrodes as anodes for the direct oxidation of urea fuel.
Here, we fabricated Mn 3 O 4 nanorod-modified ITO electrodes based on the hydrothermal process. The modified electrode was used to develop DUFCs. The fabricated Mn 3 O 4 nanostructures exhibited uniform three-dimensional nanorods with a length of approximately 1.4 μm and a thickness of approximately 100 ± 30 nm. The electrocatalytic performance of the Mn 3 O 4 nanorod-modified ITO electrode for urea electrooxidation was investigated using CV and SWV techniques at room temperature over a wide range of concentrations, from 0.2 to 4 mol/L. Moreover, the Mn 3 O 4 nanorod-modified ITO electrode has high durability, making it a promising anode for DUFC applications.

Fabrication of the Mn 3 O 4 nanorods-modified ITO electrode
Manganese oxide nanoparticles (NPs) were formed through the hydrothermal process in the presence of H 2 O 2 and KMnO 4 . Typically, 1.69 g of MnSO 4 , 3.1 g of KMnO 4 , and 35 mL of H 2 O 2 were dissolved together in 40-mL DIW. The formed solution was kept under stirring for 30 min. The ITO electrodes (1 × 2 cm) were cleaned using a basic Piranha solution, DIW, and ethanol, and then dried [29-31]. The solution was then transferred into the autoclave vessel, and the ITO electrodes were inserted horizontally inside the reaction mixture. The autoclave was closed tightly and kept in an oven at 80˚C for 2 hours. The modified electrodes were cleaned with DIW and ethanol and then dried in an oven at 80˚C.

Instruments
The Fourier transform infrared (FTIR) spectrum of the manganese oxide NPs was studied with a Nicolet 6700 Thermo Fisher Scientific spectrophotometer using the KBr pellet technique. The morphology of the manganese oxide NPs was investigated using the scanning electron microscopy (SEM) technique with a Jeol JSM-5400 LV instrument. Furthermore, the Xray diffraction (XRD) of the manganese oxide sample was studied using a Philips X-ray PW 1710 diffractometer and Ni-filtered CuKa radiation.

Characterization of the Mn 3 O 4 -modified ITO electrode
The morphology of the prepared manganese oxide was investigated using the SEM technique. The SEM image of the prepared manganese oxide (Fig 1A) depicts the formation of nanorod structures. The results showed the formation of uniform nanorods with an average thickness of 100 ± 30 nm and a length of 1.4 μm. The phases and oxidation states of the manganese oxide NPs were investigated using XRD.
where K is the particle shape factor (�0.9), λ is the X-ray wavelength (= 1.541838), β is the full width at half-maximum, and θ is the diffraction angle.

Direct electrooxidation of urea
The  (Fig 2B). These two reduction peaks could be attributed to the formation of MnOOH and Mn(II), respectively, which is in good agreement with previously reported data [43]. Fig 2C shows the electrooxidation of 1 mol/L urea in the presence of 1 mol/L of KOH, depicting a couple of redox peaks, including an oxidation peak at −0.02 V and a cathodic peak at −0.07. Moreover, the onset potential was found to be at -0.21 V (Fig 2C). The effects of different pH values on the CV response of urea at the Mn 3 O 4 nanorods/ITO electrodes were also investigated (Fig 2D and 2E). The results indicated the shifting of the redox

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peaks toward the negative direction with increasing pH values. This shifting to the negative direction suggests that this electrooxidation process is thermodynamically more favorable at higher KOH concentrations [23].
The effect of the scan rate on the electrooxidation of urea at the Mn 3 O 4 nanorods/ITO electrodes was investigated. Fig 3A shows the cyclic voltammograms of 1 mol/L at the Mn 3 O 4 nanorods/ITO electrodes under different scan rates ranging from 10 to 200 mV/s, which showed increasing redox current peaks with increasing scan rates. The relationship between the square root of the scan rates and the oxidation current peak showed a linear curve (Fig  3B). These results demonstrate that the electrooxidation of urea at Mn 3 O 4 nanorods/ITO electrodes is a revisable process.
Based on these results, all electrooxidation measurements were performed in the presence of 1 mol/L KOH at a scan rate of 100 mV/s. The performances of the developed electrode toward the electrooxidation of urea in comparison with several reported electrodes were summarized in Table 1. It is worth noting that the developed modified electrode has a lower onset potential compared with many of the reported electrodes     The results showed the appearance of a continuous, stable, and high current density. It is worthwhile to note that the resulting current was slightly decreased with time. This current decrease is attributed to the consumption of electroactive species (i.e., urea) close to the surface of the modified electrode, besides the adsorption of the gaseous byproducts onto the nanocatalyst, which reduced the catalyst efficiency [62]. Thus, the Mn 3 O 4 nanorods/ITO electrodes are stable and active electrodes toward direct urea electrooxidation. Moreover, the stability of the catalyst over the electrode was studied by investigating the morphology of the modified electrode after its use. Fig 6D showed the SEM image of the Mn 3 O 4 nanorods modified ITO after 100 cycles of its uses for electrooxidation of urea. The results showed that the modified electrode has the same nanorod morphology as that of the as-prepared modified electrode, which indicated the stability of the Mn 3 O 4 nanorods nanocatalyst over the ITO electrode surface.